Heretofore, an impeller, for example in a turbo compressor, has been rotated at a high speed by an electric motor through acceleration by a speed-up mechanism. Recently, the rotational speed of such a machine has been made as high as 30,000 r.p.m. or more. In cases where there is a need for the compressor to be compact and for the number of components to be reduced as much as possible, it is customary to provide a pinion on an impeller shaft and a gear for driving the pinion on a drive shaft disposed parallel to the pinion shaft so that the arrangement above forms a parallel driving system with two axes for the two shafts.
In the above arrangement of the gears, highly accurate meshing of gears is required if rotation is to be made at high speeds. To such end, high accuracy is required with respect to maintaining the desired distance between the two axes and parallelism between the two axes and, further, a precise tolerance is demanded in the fabrication of a gear casing, parts relating to bearings and assembly operation, etc., for example a tolerance in the range of a few hundredths to a few thousandths of millimeters with respect to the distance between the axes. (As one example, the tolerance of +0.032 -0 mm is demanded for a case where the distance between the axes is 250 mm.)
In order to assure accurate meshing of gears, it is necessary to reduce deflection of the shaft and to prevent vibration of the shaft. Therefore, endeavors have been made to increase the shaft rigidity and to select the critical speed to be substantially higher than the ordinary speed of revolution.
However, if it is intended to increase the rigidity of the shaft, the respective sizes of the shaft itself, the bearings and the gear casing would become large whereby the weight of the apparatus employing such shaft becomes heavy. In order to avoid such a tendency, a so-called flexible shaft is employed wherein the rigidity of the flexible shaft is reduced and adapted to be used at a point beyond a primary critical speed.
With the usage of the flexible shaft, the respective dimensions of the components of the apparatus would be reduced; however, upon actuation of such flexible shaft, it must pass the point of the critical speed thereof. For securely supporting the flexible shaft at the time of its passing the critical speed point and under a normal operative speed which is above the critical speed, it becomes necessary to employ a special bearing means such as a tilting pad type bearing which is a sliding bearing.
However, such a slide bearing or plain bearing is inferior in that its mechanical loss is relatively large, fabrication thereof is not easy and it is expensive. Therefore, it has been desired to employ a flexible shaft while making it possible to use a ball/roller bearing. However, in the case where roller bearings are employed for the flexible shaft, severe vibration would be produced when the rotation of the flexible shaft passes the critical speed point and the degree of vibration after passing the critical speed point may remain large. Thus, it has been considered to be impractical to employ the flexible shaft with the conventional construction using ball/roller bearings.
On the pinion shaft of the high speed side, a thrust may be imposed from the impeller or from helical gears if the pinion is of a helical gear so that an excessive force may be loaded on the bearings for the high speed side shaft which would affect the operable life, etc. of the bearings.
If the arrangements were not of a system using two parallel axes, a damper bearing would be used to solve the vibration problems in employing the flexible shaft. However, in the case where the driving relationship is established by two parallel axes, the accuracy required for the distance between the two parallel axes for transmitting torque therebetween is quite strict and, thus, it has been generally considered that employment of damper bearings which include ball/roller bearings in the two parallel axes system is difficult since use of a damper bearing in such system would permit displacement of the axis due to the load imposed in a direction normal to the axis. Accordingly, if a flexible shaft is employed in the two parallel axes driving system for transmitting torque between the axes, it has heretofore been futile up to employ ball/roller bearings for a flexible shaft because the damper bearings including ball/roller bearings are regarded as unacceptable in the two parallel axes driving system.